Generally, LPG engine systems are constructed such that LPG fuel, which has been stored in a fuel tank, is supplied into an engine after being evaporated by a mixer and a vaporizer. However, in such a conventional LPG engine system, because it is difficult for an ECU (electronic control unit) to precisely control the injection rate of fuel, the startability of the engine is reduced in the winter, and the power performance and fuel consumption ratio of the engine are decreased. As well, due to a problem of creation of tar when LPG fuel is burned, the engine is unstable when idling, and frequent engine stalling occurs.
To solve the above-mentioned problems, LPI engine systems, which directly inject LPG fuel under high pressure and in a liquefied state through an injector without having a mixer or a vaporizer, have been developed.
In this LPI engine system, because LPG fuel is directly injected through an injector and an ECU is able to precisely control the injector, the fuel consumption ratio, the power performance and the statability of the engine are improved, and vehicle maintenance to address the buildup of tar is not required.
However, the conventional LPI engine system is problematic in that, when the engine is in an ignition OFF state during a starting operation, pressure in a fuel system increases excessively due to the increased temperature of the fuel system, so that LPG fuel leaks into an intake system of the engine through the injector.
This problem is caused by the expansion in volume of LPG fuel due to the evaporation of LPG fuel, which has been in the liquid phase under high pressure, resulting from increased temperature of the fuel system when in the ignition OFF state. That is, when the vehicle travels, the LPG fuel under high pressure is maintained in the liquefied state by natural cooling by outside air, but, when the engine enters the ignition OFF state when the vehicle is stopped, natural cooling of the fuel system by outside air cannot be performed, so that the fuel system is heated through thermal transfer by high-temperature conditions in the engine compartment.
As such, if LPG fuel leaks into the intake system of the engine during the ignition OFF state, when the engine is subsequently started, because LPG fuel, which is normally injected through the injector under the control of the ECU, is mixed with and burned along with the LPG fuel which has remained in the intake system after having leaked, there is a problem in that exhaust gas contains an excessive amount of hydrocarbon.